1. Field of the Invention
The invention relates to a system and a method for purification of flue gases from power plants that work with fossil fuels, according to the oxyfuel process, and for retrofitting of power plants for preparation of the inclusion of a CO2 absorption stage (post combustion).
2. The Prior Art
Various methods and systems have been known for purification of flue gases from power plants, for a long time.
DE 43 31 415 C3 describes a device for treatment of a flue gas stream with scrubbing fluid in a container, a first spraying device for scrubbing fluid in the upper region of the container, a gas/liquid separation stage disposed below the spraying device, and a second spraying device disposed below the gas/liquid separation stage. The gas/liquid separation stage has at least multiple collection and drain gutters disposed essentially parallel at a distance from one another, which essentially extend more in the gas flow direction. The document describes the structure of an absorber used in general to purify gas, which is used for liquid cleaning of a flue gas.
DE 10 2004 061 727 A1 describes a method for treatment of sulfur dioxide/sulfur trioxide loads in the CO2 stream from a CO2-free power plant according to the oxyfuel process, whereby an additive is added to the CO2 stream. The additive minimizes the harmful effect of sulfur dioxide/sulfur trioxide.
DD 269 893 A1 describes a method and a system for additive suspension for flue gas desulfurization, whereby the suspension treatment is integrated into the flue gas desulfurization system. The additives are pneumatically conveyed into the container situated above the absorber, whereby the conveying air is guided in front of the absorption entry with an additive proportion, in a hot gas stream, and the additive suspension is applied directly to the absorber.
DD 289 211 A5 describes a method for internal sulfite oxidation in wet flue gas desulfurization systems. In the method, the air introduced into the scrubbing suspension is introduced, at least in part, not in a separate oxidation stage, but rather into the connection line between scrubbing suspension conveying pump and distribution device.
In the firing of power plants with fossil fuels, a flue gas stream occurs, which contains not only the harmful substances SO2, HCl, HF, various heavy metals, which must be precipitated out in the method, but also dust particles and mainly carbon dioxide.
Purification of flue gases from power plants fired with fossil fuels, using a scrubbing suspension based on lime/limestone, and with the addition of oxygen into the scrubbing process, to obtain gypsum products that can be used for other purposes, is known.
Scrubbing out the harmful gases from the flue gas, using a lime/limestone scrubbing suspension, takes place in various scrubber systems.
In order to achieve an optimal substance exchange between the flue gas and the scrubbing suspension, not only wet scrubbers, so-called absorbers, having different nozzle-spraying systems without contact installations, but also absorbers having nozzle-spraying systems and, in addition, contact installations having different structures are used. Also absorber systems having only contact installations are used.
For conversion of the sulfite to sulfate, oxygen is introduced, in the form of compressed minuscule air bubbles, either in a separate oxidation container, through which the limestone suspension is passed from the absorber, or, more commonly nowadays, directly into the absorber sump in which the lime/limestone suspension is kept on hand, and dispersed.
In U.S. Pat. No. 5,674,459 A1, a method is described, in which H2O2 is used as an absorption agent for sulfur oxides and nitrogen oxides, and used as an oxidation agent in place of limestone suspension or lime slurry. The products that occur in the desulfurization process are, at first, H2SO4 and HNO3. Limestone meal is subsequently used to neutralize the acid that occurs. The absorption agent used is very expensive in comparison with limestone suspension/lime slurry.
U.S. Pat. No. 5,630,991 A describes a method in which ammonium salts are used to increase the limestone meal solubility. The salts must subsequently be removed again, as ammonia, from the suspension that is passed out, in a separate reaction container, by raising the pH up to 11 to 12 with lime slurry.
U.S. Pat. No. 5,213,782 A describes a method in which magnesium oxide and quicklime are used for desulfurization. The separate reaction container that is used has the task of acting as a thickener. For desulfurization, an absorber having a spraying level is used, under which a tray is inserted.
U.S. Pat. No. 5,084,255 A describes a system for purification of flue gas from power plants fired with fossil fuels. The system consists of multiple treatment stages, whereby a suspension with magnesium hydroxide is introduced into an absorber, in the upper region, as an absorption agent added to the raw gas, and an absorption suspension sump is present in the lower region, above which the feed for the raw gas is disposed. In contrast to the method described in the present invention, here the absorber suspension is drawn off directly from the absorber sump, by means of circulation pumps, and introduced into the absorber again in the upper region, by way of spray levels, in a counter-stream to the raw gas. The absorption agent, magnesium hydroxide, which is partly recovered in the regeneration process described below, is introduced directly into the absorber. In contrast to the present invention, therefore, here only a partial stream of the absorber suspension is passed to the oxidation container, and after oxidation, it is completely transferred to the second reaction container, for precipitation with calcium hydroxide to form gypsum.
Removal of the gypsum from the process can take place only after the precipitation reaction, out of the second reaction container. The magnesium hydroxide that forms during the precipitation reaction is passed back to the absorber as an absorption agent. In the present invention, oxidation of the calcium sulfite to form gypsum, in the entire circulated absorber suspension, takes place in the first chamber. It is also passed out there, by way of hydrocyclones. The second chamber serves only as a pump reservoir for the circulation pumps and for metering in absorbents.
In the upper region, the absorber does not have any contact installation level through which raw gas flows and to which a suspension is applied. It is a further disadvantage of this system that multiple separate containers are needed for treatment, which are supplied with a partial suspension stream by way of pumps.
In U.S. Pat. No. 4,687,649, a method is described in which the flue gas is quenched in a first stage, and subsequently, in a second stage, SO2 is precipitated with a limestone meal suspension. For better oxidation of the sulfite to sulfate, here, sulfuric acid is metered in to reduce the pH, in a separate stage, and subsequently, oxidation air is introduced.
In U.S. Pat. No. 2,080,779, a method is described, in which, although a separate oxidation stage is provided, the suspension is passed to the oxidation container by way of a pump, not by way of a direct connection line between absorber and reaction container. The method serves for reducing caked-on material in the absorption and oxidation process.
DE 3136155 A1 describes a method in which a scrubbing suspension is produced from limestone meal and lime slurry. The suspension is passed to a spraying level in a scrubbing tower for desulfurization, by way of a circulation pump, whereby the flue gas flows through a gas/liquid contacting mechanism from top to bottom, in the same stream with the suspension that is nozzle-sprayed in, and leaves the scrubbing tower at the bottom, above the absorber suspension sump. The separate chambers serve for separate metering-in of limestone meal and quicklime or lime hydrate. The absorber suspension from the scrubbing tower flows into a first vessel, having an overflow to a second vessel, whereby a limestone slurry is passed to the first vessel, as an absorbent, but in substoichiometric manner, and in the second vessel, quicklime or slaked lime is added in such an amount that CO2 that forms from the first chamber (carbonic acid) is neutralized to CaCO3. The absorption liquor from the second vessel that forms in this way is passed back to the nozzles of the absorption pipe, into the absorber. A further disadvantage of this prior art is that in both vessels, no feed of oxidation air, for example via a lance, and no exhaust vapor removal are provided, and that the absorption tower functions according to the same-current principle.
In U.S. Pat. No. 5,308,509, a method is described, in which the oxidation of sulfite to sulfate takes place in the absorber sump; a partial stream of this oxidized suspension is passed to the hydrocyclone, and divided up into a gypsum-rich lower run and an upper run that contains the fine particles, flue ash, unreacted absorbent.
In EP 0 339 683 A2, a method is described, in which a partial stream of the absorber suspension has oxidation air applied to it in an oxidation container. Here, magnesium hydroxide is used as an absorption agent. In the oxidation stage, magnesium sulfite is oxidized to sulfate. The other stages serve for gypsum precipitation with lime slurry and for regeneration of the absorption agent, magnesium oxide.
In DE 29 39 744 A1, a method is described, in which chlorine and fluorine compounds are precipitated off in the first absorption stage, and a scrubbing fluid that contains calcium ions is used for the formation of gypsum, in the subsequent SO2 absorption stage. The unique feature here is that a scrubbing fluid with soluble calcium salts is used for the SO2 precipitation. To reduce the chlorine ion content, scrubbing fluid is evaporated to produce crystalline calcium chloride.
In WO 88/07023, a method for catalytic oxidation is described. The method emphasizes catalytic oxidation of sulfites in a separate container, with enzymes as catalysts.
All these systems have in common that the required oxygen is introduced into the process in the form of air, in excess. The oxygen that is not used up, and the other components of the air, as well as the CO2 that is produced during the chemical reaction, get into the flue gas, which is given off into the atmosphere subsequent to the scrubbing process. The nitrogen proportion in the flue gas is further increased by means of the oxidation feed. Furthermore, it is generally not always possible to adjust the pH of the absorption suspension in such a manner that it achieves a maximal effect in the purification process. Furthermore, the desired degree of precipitation and the desired measure of dust removal cannot be achieved with all the known methods.
In the oxyfuel process in power plants, pure oxygen is used for combustion, and a CO2-rich gas with minimal nitrogen content, but concentrated harmful gas contents, is produced. If the CO2-rich gas is freed from harmful gases and harmful substances, the concentrated CO2-rich gas can be compressed directly, for storage.